For decades, urban art forms involving spray cans, stencils and paste-ups have been executed under the cover of darkness. Often merging social commentary with urban beautification, artists needed to work quickly - and stay one step ahead of police and security guards.

Now, street art is wandering from the alley to the art gallery - thanks to star underground artists like Britain's Banksy, who auctioned a piece for 288,000 pounds (around $460,000) in 2007 and raised the eyebrows of gallerists the world over.